Filename: 118-multiple-orports.txt
Title: Advertising multiple ORPorts at once
Author: Nick Mathewson
Created: 09-Jul-2007
Status: Superseded
Superseded-By: 186-multiple-orports.txt

[Needs Revision: This proposal needs revision to come up to 2011 standards
and take microdescriptors into account.]

Overview:

   This document is a proposal for servers to advertise multiple
   address/port combinations for their ORPort.

Motivation:

   Sometimes servers want to support multiple ports for incoming
   connections, either in order to support multiple address families, to
   better use multiple interfaces, or to support a variety of
   FascistFirewallPorts settings.  This is easy to set up now, but
   there's no way to advertise it to clients.

New descriptor syntax:

   We add a new line in the router descriptor, "or-address".  This line
   can occur zero, one, or multiple times.  Its format is:

      or-address SP ADDRESS ":" PORTLIST NL

      ADDRESS = IP6ADDR / IP4ADDR
      IPV6ADDR = an ipv6 address, surrounded by square brackets.
      IPV4ADDR = an ipv4 address, represented as a dotted quad.
      PORTLIST = PORTSPEC | PORTSPEC "," PORTLIST
      PORTSPEC = PORT | PORT "-" PORT

   [This is the regular format for specifying sets of addresses and
   ports in Tor.]

New OR behavior:

   We add two more options to supplement ORListenAddress:
   ORPublishedListenAddress, and ORPublishAddressSet.  The former
   listens on an address-port combination and publishes it in addition
   to the regular address.  The latter advertises a set of address-port
   combinations, but does not listen on them.  [To use this option, the
   server operator should set up port forwarding to the regular ORPort,
   as for example with firewall rules.]

   Servers should extend their testing to include advertised addresses
   and ports.  No address or port should be advertised until it's been
   tested.  [This might get expensive in practice.]

New authority behavior:

   Authorities should spot-test descriptors, and reject any where a
   substantial part of the addresses can't be reached.

New client behavior:

   When connecting to another server, clients SHOULD pick an
   address-port ocmbination at random as supported by their
   reachableaddresses.  If a client has a connection to a server at one
   address, it SHOULD use that address for any simultaneous connections
   to that server.  Clients SHOULD use the canonical address for any
   server when generating extend cells.

Not addressed here:

   * There's no reason to listen on multiple dirports; current Tors
   mostly don't connect directly to the dirport anyway.

   * It could be advantageous to list something about extra addresses in
   the network-status document.  This would, however, eat space there.
   More analysis is needed, particularly in light of proposal 141
   ("Download server descriptors on demand")

Dependencies:

   Testing for canonical connections needs to be implemented before it's
   safe to use this proposal.


Notes 3 July:
  - Write up the simple version of this.  No ranges needed yet.  No
    networkstatus chagnes yet.

